What would cause Cylinder #3 misfire?

What is your question?

My check engine light came on and I had the codes ran and it came up 'Cylinder 3 misfire'. It has been rough idling, using oil and terrible gas mileage. This weekend, I replaced the valve cover gasket, plugs, and coils. Reset the check engine light however, 5 minutes down the road it came back bright and blinking. Any suggestions before I breakdown and take it to someone to run diagnostics?